1. Field of the Invention
This invention relates to apparatus for inspecting flat glass for defects. More especially the invention relates to apparatus for the continuous inspection of a moving ribbon of flat glass for example a ribbon of annealed float glass which is approaching a cutting station.
2. Description of the Prior Art
The continuous inspection of the moving ribbon of flat glass travelling on a roller conveyor has been proposed by employing apparatus which includes a light source which projects light at an angle through the whole width of the ribbon of glass. The light source is positioned above the ribbon and the light transmitted through the glass is received on a screen which is positioned beneath the ribbon and is substantially horizontal.
To ensure continuous illumination of the screen there is a gap in the roller conveyor equivalent to two of the conveyor rollers. The screen is positioned beneath this gap and receives the light transmitted through the glass. Also an examiner seated above the conveyor just downstream of that gap has an uninterrupted view through the ribbon of the screen on which images of defects in the ribbon of glass appear as shadows.
Thus defects which cast shadows on the screen are monitored visually by the examiner who marks the ribbon where the defect exists using an ink marker on the end of a pointer. The sensitivity of detection of defects of different sizes is adjusted by raising or lowering the screen relative to the glass in the gap in the conveyor.
This inspection apparatus has the disadvantage that at high ribbon speeds the examiner has only a very short time, for example 2 to 3 seconds, to identify, locate and mark a defect. A significant level of unmarked defects remain in the glass. This makes necessary further visual examination of the glass after it had been cut into sheets at the cutting station. This is a particular problem when glass of high quality is required.
It is not usually practicable to reduce ribbon speed in order to increase viewing time when there are high production demands since this would give rise to an increase in production costs.
Viewing time might be increased by removing more than two rollers from the conveyor to provide the viewing zone and thereby increase the length of the ribbon under examination. This is hazardous however since the length of the unsupported ribbon is increased which can lead to breakages in the ribbon with disruption of production.
In this prior method the maximum sensitivity available by lowering the screen is limited for example to a magnification factor of about 1.4. Generally the magnification is non-linear across the width of the screen.
It is a main object of the present invention to provide an improved inspection apparatus and to improve the working conditions for examiners.